The Kane computer1 requires single donor 31P atoms to be placed in an ordered 1D or 2D array in crystalline silicon. The atoms must be separated from each other, by 20 nm or less. An alternative architecture is that of Vrijen et al.2 who propose an array of 31P atoms in a heterostructure where the atom spacing can be larger than the Kane computer but still of the order of 100 nm. Such precise positioning has proved extremely difficult using conventional lithographic and ion implantation techniques, or using focused deposition. This difficulty is not only with regard to forming arrays of donor atoms with sufficient precision, but also ensuring that only single donor atoms have been introduced into each cell of the array.
Optical lithography has been utilised by semiconductor industries to manufacture integrated circuits with great precision. Optical lithography systems include an exposure tool, mask, resist and processing steps to accomplish pattern transfer from a mask, to a resist, and then to a device. However, the use of resist layers can limit resolution to the wavelength of the radiation used to transfer the pattern in the mask onto the resist. This is presently about 100 nm.
Electron beam lithography, which uses a finely focused electron beam to directly write patterns into resists, can attain better than 20 nm resolution. Further, the “top-down-process”, described in a recent patent application, uses electron beam lithography to construct arrays of nanoscale channels in resists. The resist is then irradiated with an ion beam so that ions impact at random on the surface allowing a random array of channels to direct one or more atoms through into the substrate to construct nanoscale structures.
However, in all of these lithographic techniques, control of the number of atoms reaching the substrate is not possible.
Lüthi et al.4 describe a resistless lithography technique which enables the fabrication of metallic wires with linewidths below 100 nm. The technique is based on an ultra-high resolution scanning shadow mask, called a nanostencil. A movable sample is exposed to a collimated atomic or molecular beam through one or more apertures in an atomic force microscope (AFM) cantilever arm. Standard V-shaped Si3N4 cantilevers with integrated tips having a spring constant below 0.1 Nm−1 were used. The aperture diameter ranged from 50 to 250 nm depending on the desired mask structure. Scanning the sample with respect to the nanostencil allowed the structure to be laid down on the surface of the sample. After nanostructuring, the structure was inspected with the AFM tip.
This former method allows precise positioning of large numbers of atoms but not implanting and detecting single ions.
Shinada et al.5 have developed a single ion detection technique using a single ion implantation assembly developed by Koh et al.6 The single ion implantation assembly consisted of a pair of deflector plates, an objective slit, a precision quadropole-magnet, a target, an electron multiplier tube (EMT) and a chopper control circuit connected to the deflector plates and the EMT. The ion beam is chopped with the pair of deflector plates, over which the potential difference can be switched. Each single ion is extracted one by one from a continuous ion beam by adjusting the ion beam current, the objective slit diameter and the switching time of the potential difference applied to the deflector plates.
The extracted single ion is then focused with the quadropole-magnet lens and impacts on the target. The number of incident ions is controlled by the EMT by detecting secondary electrons emitted upon ion incidence. Signals from the EMT are fed to the chopper control circuit which keeps on sending the beam chopping signals to the deflector until the desired number of single ions are detected.
Shinada et al.5 emphasised findings by Koh6 by reporting that the key to controlling the incident ion number is the detection of secondary electrons emitted from a target upon ion incidence.
The secondary electron detection efficiency Pd is defined as follows:             P      d        =                  N        SE                    N        ext              ,where NSE is the average number of detected secondary electrons by a single chop and Next is the average number of extracted ions by a single chop, where Next is proportional to the ion beam current and the time of beam chopping.
To determine the efficiency in the determination of secondary electrons, a 60 keV Si2+ ion beam was chopped with a frequency of 100 kHz. NSE was estimated by dividing the number of secondary electron counts per second by 105. To evaluate Next, a standard fission track detector was used.
The secondary electron detector included a photomultiplier tube with a scintillator and a light guide. A grid electrode was used to guide the secondary electrons to the sensitive part of the scintillator.
The experimental result for Pd was 90%. The error was partially attributed to the limitations of the secondary electron detection system. Furthermore, results showed that the single ion incident position could be successfully controlled with an error of less than 300 nm.
This detection of impacts from the pulse of secondary electrons emitted from the surface due to the ion impacts does not distinguish ion impacts with a mask from ion impacts with an exposed substrate under the mask.